This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-83914 filed on Mar. 24, 2000.
1. Field of the Invention
The present invention relates to a motor having a rotor equipped with a cooling fan.
2. Description of Related Art
one previously proposed rotor of a motor has a rotatable shaft to which an armature, a commutator and a cooling fan (hereinafter simply referred to as a fan) are secured. The rotor is substantially contained in a yoke. The yoke has a plurality of vent holes that communicate an interior of the yoke with an exterior of the yoke. These vent holes are disposed adjacent the fan. When the rotor rotates, the fan is rotated integrally with the rotor to circulate the air for cooling an interior of the motor.
In one type of such a motor, the number of slots formed in a core of the armature, the number of blades of the fan and the number of vent holes are set to twelve, six and eighteen, respectively. In such a motor, vibrations are generated as shown in FIG. 8.
More specifically, due to the fact that the number of slots is twelve, vibrations generated during movement of the slots across a magnetic field (for instance, a magnetic field generated adjacent magnets that are secured to the yoke) show peaks at 12 Hz (primary vibration), 24 Hz, 36 Hz, and so on. Furthermore, due to the fact that the number of blades is six, vibrations generated on the blade side during movement of the blades over the adjacent vent holes show peaks at 6 Hz, 12 Hz, 18 Hz, 24 Hz, 30 Hz, 36 Hz, and so on. Also, due to the fact that the number of vent holes is eighteen, the vibrations generated on the vent hole side during the movement of the blades over the adjacent vent holes show peaks at 18 Hz, 36 Hz, and so on.
All of these vibrations (i.e., vibrations generated at the slots, the blades and the vent holes) are added together (i.e., resonate together) in the motor at 36 Hz, 72 Hz, and so on, generating particularly large vibrations at these frequencies. As the number of these resonance frequencies at which all of the vibrations are added together (i.e., resonate together) increases within a predetermined frequency range (audible frequency range of generally between 16 Hz and 20 KHz), a noise level of the motor disadvantageously increases.
Thus, it is an objective of the present invention to reduce a noise level of a motor that has a rotor equipped with a cooling fan.
To achieve the objective of the present invention, there is provided a motor including a rotatable shaft, a core secured to the rotatable shaft, a fan secured to the rotatable shaft, and a yoke containing the core and the fan. The core has a plurality of slots around which a coil is wound. The fan has a plurality of blades. The yoke has a plurality of vent holes that communicate an interior of the yoke with an exterior of the yoke. The plurality of the vent holes are arranged adjacent the fan. The number of slots, the number of blades and the number of vent holes can be set such that the number of slots, the number of blades and the number of vent holes do not share a common divisor other than 1. Instead of setting all of the three numbers in the above manner, only the number of slots and the number of vent holes can be set such that the number of slots and the number of vent holes do not share a common divisor other than 1. Furthermore, only the number of blades and the number of vent holes can be set such that the number of blades and the number of vent holes do not share a common divisor other than 1.